Electric utility power distribution systems are frequently constructed underground for a variety of reasons ranging from objections to the above-ground aesthetics, the premium of above-ground space in dense urban locations, and safety concerns. Accordingly, power distribution systems heretofore constructed of poles, wires, and pole-mounted switches and transformers are being superseded and even replaced by underground systems in underground installations.
Whether used in overhead or underground locations within a power distribution system, the main function of current-interrupting switches is to isolate desired sections to allow for maintenance. While overhead space is relatively open and unrestricted, space in underground installations is at a premium. Underground installations (which are also referred to as “vaults”) are relatively small and need to have enough space for all the necessary material, as well as enough room for lineman to safely work inside.
For many vaults, switch installation requires using an equipment access hole which may require lifting a heavy cover and can be costly. A switch that can fit through a maintenance hole (sometimes referred to as an “access hole” or a “manhole”), however, can be very cost effective. Many switches currently used in underground vaults contain oil or SF6 gas as an electrical insulation medium in order to make the switch small. It is possible that a switch containing oil or SF6 gas can be made small enough to fit through a maintenance hole; however, rising environmental and safety concerns discourage the use of oil and SF6 gas, which can each be flammable and/or explosive while presenting environmental hazards when leakage occurs or when emissions are created. Thus, utility companies are trying to move away from switches with oil or SF6 gas.
Three-phase vacuum switches have been manufactured under the Elastimold trademark by Thomas & Betts Corporation (Memphis, Tenn.) that fit through a maintenance hole, and utilize vacuum interrupter bottle switches as the current-interrupting switch. The vacuum interrupter bottle switches utilized in the Elastimold switches are molded inside a rubber housing and surrounded with a thin metal sheet. Vacuum interrupter bottle switches are manufactured so that the inside components cannot be seen. The only indication as to whether or not the switch is opened or closed is the position of an exterior handle which is not the most direct type of visible evidence one wishes to have when dealing with such high voltages and currents. It is not possible to determine whether the switch is truly open or closed because the movable contacts are hidden inside.
Three-phase vacuum switches have been developed to fit through a maintenance hole, and utilize vacuum interrupter switches as the current-interrupting switch. However, they do not incorporate a visible disconnect switch as a safety feature. It is not possible to determine whether a vacuum interrupter bottle switch is open or closed because the movable contacts associated with the switches are contained within the sealed body of the bottle.